Decoding Spay/Neuter Research, Part 1

Over the last couple years, you may have read about some of the studies that claimed to find negative health consequences of neutering (including spaying) of certain breeds of dogs. Perhaps your clients or vets in your community have raised objections to pediatric neutering—or neutering at all. This blog is the first of two in which I will review some of the limitations of these studies and hopefully provide you with some ammunition to defend your neutering practices if you encounter objections.

The health effects of neutering is a topic that I am passionate about. After working in veterinary practice in the 1990s, I decided to pursue a PhD in epidemiology at Cornell University. By the time I graduated, I had spent 4 years leading research on long-term risks and benefits of pediatric neutering and publishing the findings in reputable peer-reviewed journals. I learned that conducting high-quality epidemiologic research on this topic is hard work, but it can be done.

Unfortunately, most of the studies from the last couple of years were subject to serious flaws that make the findings unreliable for understanding the risks and benefits of neutering dogs. To complicate matters, the subsequent social media coverage came dangerously close to being fake news by cherry-picking findings that would support an anti-neuter perspective and ignoring the other findings. For illustrative purposes, I will focus on a study of German shepherds led by Dr. Benjamin Hart, highlighted last year in the Journal of the American Veterinary Medical Association and whose associated webpage was widely shared on social media.

The first problem I’d like to highlight is what happens when you have data on only those dogs seen at a specialty referral center—such as the UC Davis Veterinary Medical Teaching Hospital (UCD VMTH)—when trying to measure how often a medical problem occurs in dogs. Suppose you surveyed patrons at a 5-star restaurant in Maine and found 75% of people eating lobster and 0% eating French fries. Does that mean most Americans are eating lobster every day and they’ve all stopped eating fries? Of course not. Those findings tell you more about who eats at that particular restaurant and what’s available on the menu. We have a similar problem with selection bias in the recent neutering studies based on UCD VMTH cases. Dogs seen at that particular hospital would often be those whose owners sought out the specialized treatments offered for unusual or severe medical problems, which means those conditions will look more common. On the other hand, health problems usually treated by a regular vet would be seen less often at this hospital and would artificially appear to be uncommon. Mammary cancer, for example, is a relatively common problem in sexually intact female dogs, but Dr. Hart reported that mammary cancer was diagnosed in only 4% of intact females. The most plausible explanation is that the dogs with mammary cancer were often treated by their regular vets and never referred to this hospital. This bias in which types of medical conditions were referred to this hospital likely has the effect of making the intact females appear at lower risk for mammary cancer than they actually are.

In part 2 of this blog, I will walk through some other flaws of these studies and the ensuing social media coverage—and then provide some suggestions for what to do when you see reports on a new study about neutering or are discussing those studies with clients or community vets.

How does your organization approach spay/neuter of shelter pets—and services provided to your local community?

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Vic Spain, DVM, PhD

ASPCA Senior Director, Applied Research

Dr. Vic Spain’s current research includes effectiveness of animal-related legislation, assessing state and local emergency preparedness for animals and methods for connecting consumers to higher welfare animal-based food products. Dr. Spain received his Doctorate of Veterinary Medicine from the University of California, Davis and later attended Cornell University, where he received a PhD in Epidemiology with a graduate minor in biostatistics.